Interactions Between Ferroelectricity and Superconductivity in SrTiO3

Salva Salmani-Rezaie, Timo Schumann, Luca Galletti, Kaveh Ahadi, William Strickland, and Susanne Stemmer

Materials Department, University of California, Santa Barbara

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MISPOCHE-2020

May 4, 2020

Phase Transitions in Doped, Strained SrTiO3 Films

Paraelectric ➜ Ferroelectric ➜ Superconducting

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K. Ahadi et al., Sci. Adv. 5, eaaw0120 (2019).R. Russel et al., Phys. Rev. Mater. 3, 091401(R) (2019).

LSAT

R:SrTiO3

Phase Transitions in Doped, Strained SrTiO3 Films

Paraelectric ➜ Ferroelectric ➜ Superconducting

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● Superconducting transition temperature is enhanced by a factor of two compared to unstrained films (Tc > 600 mK)*

● Superconducting dome is unaffected● Ferroelectric transition temperature decreases with

doping concentration● Ferroelectric and and superconducting transitions

“collapse” at a similar carrier density

K. Ahadi et al., Sci. Adv. 5, eaaw0120 (2019).R. Russel et al., Phys. Rev. Mater. 3, 091401(R) (2019).

*See also: Tomioka et al., Nat. Comm. 10:738 (2019); Van der Marel, APS 2019; Stucky et al., Sci. Rep. 6, 37582 (2016); Herrera et al., Phys. Rev. Mater. 3, 124801 (2019); Rischau et al., Nat. Phys. 13, 643 (2017).

LSAT

R:SrTiO3

Nature of the Ferroelectric Transition

Paraelectric ➜ Ferroelectric

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Prototype displacive/softmode phase transition?

LSAT

R:SrTiO3

➜

Polar Nanodomains in the Paraelectric Phase

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LSAT

SrTiO3

20 pm

15

10

5

0

20 pm

15

10

5

0

SrTiO3

SrTiO3

20 pm

15

10

5

0

● Static Ti-column displacements and polar nanodomains in the paraelectric phase of strained SrTiO3 films

● Local order, global disorder● Coulomb interactions cause

displacements already in the paraelectric phase

● Order-disorder transition● Domains grow as temperature

approaches the transition and reorient for a globally polarized phase

Polar displacement

Polar vector orientation

S. Salmani-Rezaie et al., submitted.

Polar Nanodomains in the Paraelectric Phase

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LSAT

SrTiO3

Local ferroelectric order in the paraelectric phase

SrTiO3

”…low-temperature SrTiO3 is locally ferroelectric already at zero stress.”

S. Salmani-Rezaie et al., submitted.

Polar Superconductor + Spin Orbit Coupling

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● Breaking of inversion symmetry makes spin-orbit coupling relevant● Singlet–triplet mixing expected, but details matter● Topological superconductivity and unconventional pairing often occur together*● Proposals of polar fluctuations mediating odd-parity/topological and unconventional SC

● Experimental signatures? * For LAO/STO interfaces, see Scheurer et al., Nat. Commun. 6, 6005 (2015).

Experimental Signatures of Odd-Parity SC

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700

600

500

400

300

200

100

0

T c (m

K)

2015105

n3d (×1019 cm-3)

A

B

C

D

LSAT

SrTiO3 8

6

4

2

0

µ 0H

c2/T

c (T/

K)

98765

n3d (×1019 cm-3)

AB

C

D

IP OOP

● Critical fields above the Pauli limit seen in most films

● Usually taken as a strong indicator of odd-parity SC

● For odd-parity states, there is no suppression of superconductivity if the field is along with the direction of Cooper-pair spin

● Other explanations possible, e.g. multiband SC

LSAT

R:SrTiO3

Sample D is has an interfacial layerSamples B and D are partially strain-relaxed

Surface depletion of ~ 150 nm thin films causes rapid drop in Tc at low doping

T. Schumann, L. Galletti et al., Phys. Rev. B 101, 100503(R) (2020)

Experimental Signatures of Odd-Parity SC

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γ is the nonreciprocity and is predicted to be enhanced in case of triplet pairing in a theory of two-dimensional Rashba superconductors

”Nonreciprocal current” 𝑅 = 𝑅! 1 + 𝛾𝐵𝐼 𝑉 = 𝑅𝐼 + 𝛾𝑅𝐵𝐼" Only for I ⊥ B

R. Wakatsuki et al., Sci. Adv. 3, e1602390 (2017).R. Wakatsuki et al. Phys. Rev. Lett. 121, 026601 (2018).

T. Schumann, L. Galletti et al., Phys. Rev. B 101, 100503(R) (2020)

Sample B Sample D Sample D

Large nonreciprocal current in partially-strain relaxed films.No signal in other films.

Role of polar fluctuations?

Summary and Open Questions

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● Signatures of odd-parity SC in doped, polar SrTiO3 films● Need to be better understand role of spatial nonuniformity, polar

fluctuations, 2D, …● Suppression of polar nanodomains with doping in the paraelectric

phase is key to suppressing the ferroelectric transition● This increases the superconducting transition temperature, until

disorder destroys both